This proposal will test the hypothesis that peptides can be presented to T cells via novel MHC/immunoglobulin fusion proteins, and will apply such fusion proteins to the treatment of disease. The studies will merge two areas of investigation: 1) soluble MHC; 2) our findings related to anti-T cell receptor (TCR) monoclonal antibodies (mAbs), generated during the first cycle of this grant. Soluble MHC can activate peptide- specific T cells in vitro, but have generally not been effective at inducing T cell response in vivo due to lack of multi-valent TCR cross- linking, lack of co-stimulation, and short in vivo half-lives. Anti-TCR mAbs induce multi-valent TCR cross-linking, can induce co-stimulation by cross-linking accessory cells, and have long in vivo half lives. Mitogenic, Fc receptor (FcR) cross-linking, forms of mAb stimulate T cells in vivo, while non-mitogenic, FcR non cross-linking, forms down- regulate T cells. However, these mAbs induce undesirable global effects, and would be more useful if they could be targeted to peptide-specific T cell clones. To combine the desirable characteristics of soluble MHC with anti-TCR mAbs, we have generation fusion proteins in which the extracellular domains of class I or class II are joined to hinge, CH2 and CH3 domains of IgG1 (FcR cross-linking) and IgG3 (FcR non cross- linking). In Aim #1, class I (K/b/IgG) and class II (IA/d/IgG) molecules designed to optimize their stability and signaling properties will be evaluated. We will determine the effects of the fusion proteins on peptide-specific naive and memory CD4+ and CD8+ T cell responses in vivo, and how that fate is influenced by duration of TCR signaling, degree of TCR cross-linking, B7-mediated co-stimulation, and co- administration of inflammatory cytokines. In Aim #2, we will test the hypothesis that MHC/Ig fusion proteins can be applied to the treatment of experimental disease. We will determine whether K/b/IgG fusion proteins can function as tumor vaccines using a murine Kb-restricted, peptide-specific tumor model. We will determine whether IA/q/IgG fusion proteins can inhibit autoimmunity using the IAq-restricted, type II collagen-specific collagen-induced arthritis model. These studies may lead to strategies useful for modulating the immune system in a variety of clinical disease states.